I currently began a new project with processing and first started with the processing-ide that comes together with the installation. Is it possible to structure the project into subdirectories instead of having to put all classes into the single sketch folder? I looked at the documentation at https://processing.org/ but didn't find any useful information about how to divide projects into manageable source subdirectories. Your help is much appreciated :)
No the default Processing IDE does not support this.
Before switching to another IDE for Processing, keep in mind that Processing 3 will highly focus on creating an entirely new IDE, which "might" add this functionality.